Along with diversification of energy resources, power generation through use of a solar photovoltaic panel has become widespread as a type of renewable energy. Against the backdrop of lack of energy sources and reduction in CO2 emission in recent years, large-scale solar photovoltaic power generation plants, each of which exceeds 1000 kW and is called mega solar, have been actively built. Hereinafter, solar power generation is also called PV (Photo-Voltaic), and a solar photovoltaic panel (solar photovoltaic power generation panel or solar panel) used therefor is also called a PV panel.
Power output of a PV panel (hereinafter represented as output power, or simply represented as output) varies according to the amount of irradiation light (amount of solar irradiation), i.e., the magnitude of received light energy. In particular, when the amount of light is small, such as at dawn, the output is small and the internal impedance is high. Connection of a load in a state with a high internal impedance reduces the voltage, which prevents normal operation as a power source, and the power source becomes an unstable power source. Thus, control for stable operation even in a state where a PV panel receives a low amount of light is required. Such control is called optimization. Optimization means (circuit or apparatus) is called an optimizer. In some cases, the configuration of what is called a power conditioner may include an optimizer.
FIG. 9 is a diagram illustrating the amount of generated power by a PV panel against variation in amount of solar irradiation a day. Effects of cloudiness and movement of cloud are not considered. The graph of FIG. 9 qualitatively shows the generated power of PV panel in an ideal solar radiation condition in a clear sky with sunrise at 6 o'clock and sunset at 18 o'clock. The PV panel starts to be irradiated with sunlight at sunrise, and power generation gradually starts. Although there is some variation according to the installation latitude of PV panel, orientation to the sun, and the installation inclined angle of the PV panel, the amount of solar irradiation under a completely clear sky in midsummer is the maximum at about 13 o'clock. Subsequently, the amount of generated power gradually decreases, and becomes substantially zero after sunset.
The amount of generated power according to variation in amount of solar irradiation as described above and variation in season and weather is required to be controlled so as to be the maximum. This control is optimization. Typically, along with means called a power conditioner, maximum power point tracking is executed for this control.
FIG. 10 is a diagram illustrating the maximum power point tracking control. The abscissa axis of FIG. 10 indicates the output current (I) of PV panel, and the ordinate axis indicates the output voltage (V) of the panel. A curve P indicates variation in the generated power (P) of the PV panel against variation in the output current (I) and output voltage (V) of the PV panel. The generated power (P) is output voltage (V)×output current (I). The generated power (P) in a case where the current is A1 and the voltage is V1 is represented as an area A, and a point A on the curve P represents a power operating point (power point).
Likewise, the power (P) in a case where the current is A2 and the voltage is V2 is represented as an area B, a point B on the curve P is represented as a power operating point. The power (P) in a case where the current is A3 and the voltage is V3 is represented as an area C, and a point C on the curve P is represented as a power operating point. The maximum generated power in FIG. 10 is in the case where the current is A2 and the voltage is V2 represented by the power operating point B with the area of the output voltage (V)×output current (I) being the maximum. Selection of the current and voltage of the PV panel to cause the power operating point to be B is maximum power point tracking control (MPPT).
FIG. 11 is a diagram illustrating the relationship of generated power of PV panel with a parameter of the amount of solar irradiation. The abscissa axis indicates the voltage (V), the left ordinate axis indicates the current (A), and the right ordinate axis indicates the power (W). In the diagram, curves S1 to S5 are characteristic curves (i.e., voltage and current curves for respective amounts of irradiation) that represent variation in output voltage−output current of the PV panel for amounts of irradiation (Incident Irrad.=W/m2) of the PV panel, which serve as parameters. The curve S5 is a voltage and current curve at the maximum amount of irradiation. Symbols o (M1 to M5) assigned to the respective curves S1 to S5 indicate the amounts of irradiation of PV panel. The maximum power point of the curves S1 to S5 is also indicated. FIG. 11 is for illustrating a measurement example in a case where the solar panel temperature is 25° C.
The curve P5 represents the characteristics (voltage and power curve) of the output power at the maximum amount of irradiation S5 (W/m2) at an installation site of the PV panel. The characteristics of the output power of the amount of irradiation S1 to S4 (W/m2) at the installation site of the PV panel are also drawn as a curve analogous to the curve S5. The curve is, however, omitted in FIG. 11. Points M1 to M5 indicate the maximum point of voltage (abscissa axis)×current (ordinate axis)=power for the respective amounts of irradiation. The point M5 indicates the maximum point of voltage (abscissa axis)×current (ordinate axis)=power on the curve S5. The point A5 indicates the maximum point of power on the curve P5.
In FIG. 11, the maximum generated power at the maximum amount of irradiation S5 (W/m2) at the installation site of the PV panel is indicated as the point M5 (symbol o). This point M5 (symbol o) represents the maximum power point A5 (symbol ⋄) of the curve P5. In FIG. 11, the power value, which is the value of M5 on the ordinate axis (left scale: current value)×the value on the abscissa axis (lower scale: voltage value), coincides with the power value, i.e., the value of A6 on the ordinate axis (right scale: power value). When the output current of the PV panel is higher than the output current at the point M5 (=low output power: left side on the sheet of FIG. 11) or lower (=high output power: right side on the sheet of FIG. 11), the output power is lower than the maximum output power point A5 (symbol ⋄) as represented by the curve P5.
Disclosure of such types of conventional arts includes Patent Literature 1, Patent Literature 2, Patent Literature 3, Patent Literature 4, Patent Literature 5, Non Patent Literature 1 and Non Patent Literature 2.